The measurement of the absolute value of intracranial pressure (ICP) is important in diagnosing and treating various pathophysiological conditions caused by head trauma, hemorrhage tumors, inflammatory diseases and the like. Several techniques have been used to measure ICP. Conventional invasive ICP measurement techniques require surgical passage through the skull bone into the brain ventricles, parenchyma or the region between the skull and dura matter to implant a measuring transducer. Such invasive techniques, however, are undesirable, as damage to the sensitive brain tissues may result. Moreover, due to the invasive nature of the procedures, infections may be caused despite precautions, which infections can be serious or even deadly.
Various systems and methods for the noninvasive measurement of ICP have been suggested. Several of these techniques rely upon ultrasonic imaging to detect relative displacements of tissue boundaries within the brain. The displacements may be associated with fluid build-up and compression or dilation of brain vessels, which permits determination of ICP through an independent calibration of compressibility. An alternate noninvasive ultrasonic technique involves the measurement of blood flow in the carotid artery by ultrasonic excitation of the artery and determination of Doppler frequency shift. Examples of various types of ultrasonic measurement techniques are disclosed in U.S. Pat. No. 5,074,310 to Mick and U.S. Pat. No. 5,117,835 again to Mick, U.S. Pat. No. 5,388,583 to Ragauskas et al., U.S. Pat. No. 5,411,028 to Bonnefous, U.S. Pat. No. 5,617,873 to Yost et al., and U.S. Pat. No. 5,919,144 to Bridger et al. Each of the patents cited above is incorporated herein by reference.
Other noninvasive ICP measurement techniques that have been proposed involve determining the displacement of various tissues or body members. For example, it has been proposed, that ICP can be measured by observing the tympanic membrane of the ear. (See Buki B. et al., Otoacoustic Emissions: A New Tool For Monitoring Intracranial Pressure Changes Through Stapes Displacements, Hear Res 1996 May; 94 (1–2): 125–39; Buki B. et al., Middle Ear Influence On Otoacoustic Emissions—II: Contribution Of Posture And intracranial Pressure, Hear Res. 2000 February; 140 (1 –2):202–11; Reid A., Marchbanks R. J. et al., The Relationship Between Intracranial Pressure And Tympanic Membrane Displacement, British Journal of Audiology, 1990: 24:123–129; U.S. patent Publication No. 2001/0027335 A1). It has also been proposed that ICP in infants can be measured by observing the fontanelle. (See U.S. Pat. No. 4,995,401).
However, all of these proposed systems and methods suffer from at least one significant disadvantage. More specifically, while such systems and methods can be used to measure changes in ICP, such systems and methods can not be used to determine the absolute value of ICP without performing some type of calibration (for example, using known invasive techniques). This is true because displacement of various internal and external tissues or body members in response to various degrees of ICP is not the same from person to person. For example, the degree of distension of the tympanic membrane of the ear in one patient having a certain ICP may be completely different than in another patient having the exact same ICP due to numerous factors such as age, size, thickness of the tympanic membrane, degree of dehydration, etc. Thus, without performing some type of calibration for each patient, the absolute value of ICP for that patient can not be determined using the above methods. The dilemma is that if an individual noninvasive ICP absolute value measuring device exists with acceptable accuracy for calibration of other noninvasive ICP meters, the other meters are unnecessary.
A noninvasive ICP absolute value measuring method and apparatus is presented in U.S. Pat. No. 5,951,477 to Ragauskas et al., which is commonly owned with the present application, the entirety of which is incorporated by reference herein. This patent discloses the use of an ultrasonic Doppler device which detects the velocities of the blood flow inside the optic artery for both intracranial and extracranium optic artery portions, which velocities are used to calculate an absolute ICP. However, it would be more desirable if a simpler technique could be developed.
What is desired, therefore, is a system and method for determining the absolute value of ICP which is non-invasive, which can be used without performing calibration thereof, which is relatively simple to use, and which is safe and dependable.